This invention relates to an image forming apparatus that forms images by means of an electrophotographic process using an exposure unit for exposing an image carrier based on image data.
Conventionally, in an image forming apparatus such as a digital copier, a printer, a facsimile apparatus, there has been known a solid-scanning head (hereinafter referred to as LED head) such as an LED array as an exposure device (exposure unit) for exposing an image to a photoconductor. Compared with an optical-scanning device such as a laser (hereinafter referred to as LD head), the LED head is superior in properties of a smaller number of moving parts, a high degree of reliability and a construction that can be miniaturized, and save spaces. The LED head is driven based on image data generated from document information that is read out by an image reader in the image forming apparatus, or image data that is transmitted from outside of the apparatus, thereby emitting light, exposing the photoconductor, and forming an electrostatic latent image. The electrostatic latent image is developed for forming an image on a sheet.
This kind of the image forming apparatus is desired to produce images without color deviation in color image forming operations. Especially, in a tandem system that images formed in an image forming unit for each of colors are multi-transferred onto single transfer body (such as transfer belt), it is necessary to detect an error between image forming positions of each of units, and to correct the image data. For this purpose, a resist mark or pattern predetermined for each of units is formed on the transfer body, and optically read out by a detector, thereby adjusting the positions.
For example, there has been provided with an art for correcting image positions in a color image forming apparatus using separated LED heads for four colors, as disclosed in Japanese patent application publication No.10-315545 in which the apparatus is comprises sensors for detecting positional deviation, that are located at both sides of the transfer belt thereof, and corrects deviation of colors (CMY) with respect to a reference color (K) based on the detection by the sensors.
Here, referring to FIG. 18, the explanation is given to a method for detecting color deviation in an image forming apparatus using a conventional LED head. The apparatus is equipped with optical sensors S1, S2 and S3 that are located in front, middle and back sides of a transfer belt V in a main-scanning direction (LED scanning direction) in a downstream side from an image forming unit, and the resist determining patterns are formed on the transfer belt V in a sub-scanning direction, wherein each of the sensors determines the resist determining patterns. Each of patterns of black (K), cyan (C), magenta (M) and yellow (Y), consisting of horizontal lines and oblique lines, is formed at a predetermined interval. Then, the pattern images are read out for detecting deviation amounts of the cyan, magenta and yellow image patterns with respect to the reference K image pattern. Based on the detected value, the deviation amounts of C, M and Y with respect to the reference color K is corrected by approximating to a curve of the second order. The correcting method is carried out by storing correcting coefficient data to a memory, and executing control of drawing timing in response to the data. A curve shown in FIG. 18 indicates curvature (bow) occurring in the image.
It should be noted that, as shown in FIG. 19, elements of the color deviation include deviation in the main-scanning direction, in sub-scanning direction, in angles (skew), scanning lines (bow) and scaling ratio in the main-scanning direction.
The above method for correcting the color deviation by the conventional three-sensor structure, has problems such as (1) the cost increases, (2) the processing system is complicated, (3) approximative errors occur, and (4) if the method is applied to the image forming apparatus using the LED head, linear distortion of the LED exposure unit itself cannot be detected. Further, in the image forming apparatus using the LED head (exposure unit), since bow distortion and deviation in main scaling ratio due to characteristics of optical systems do not occur, only deviation in the main/sub-scanning directions, and skew are required to be corrected, so that correction of color deviation can be corrected by means of a two-sensor structure, that are simple and low cost.
However, the LED exposure unit has a problem of the linear distortion of the LED unit itself caused by distortion (during manufacturing) in linear property of LED arrays (solid scanning exposure device), or distortion during assembly of the exposure unit of the LED arrays. This distortion occurs in a curvature shape, so that the sensor cannot detect it. Besides, the linear distortion in the exposure unit itself may have a curvature profile of the high order, which cannot be handled even by a three-sensor structure. In order to detect such a distortion, greater numbers of sensors are required, which complicates the structure or detection algorithm. As a result, the simple structure cannot have solved the problem of the color deviation caused by the solid scanning exposure device.